I was using a gray card when shooting a series of photos in a forest, because I found the light a bit tricky there. The plan was to set the white balance of the raw photos later in post processing, but the problem is that the color of the gray card appears uneven.

Depending on which spot I measure I get anything between 5900K and 7000K, with very different results on the overall picture obviously. When I shot the card, I defocused the lens to get an even color, and then applied color noise reduction all the way in Lightroom, but I still can't get a clear reading. Anyone have experience in how to do it?

I can't answer for the gray card part, but I frequently shot in forests. Personally, I use one of camera's presets acording to what the sky is: Sunlight, Overcast (Or Shade in some situations, like way after the sunset of before sunrise) My preferred is Sunlight(5300-5500k). Also, set low contrast (0 or -1 in a camera that allows -2 to +2) and saturation up (in 0 or +1) These settings work best for me, as they give me "neutral" looking images that I can easily adjust in post. I think that records correctly the color casts, so I can compensate adequately after the fact.
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JahazielOct 2 '12 at 15:13

2 Answers
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Gray card is used to make your colors neutral from the tone of a light source. When you have multiple light sources with different color temperatures (such as sunshine, blue sky lighting the shadow areas, and reflections colored by surrounding objects) you can only pick one of them to be the "neutral" light.

So if you have multiple light tones, it's your creative choice which light you want to consider "neutral". Often, the main light is chosen to be neutral. In daylight, this gives blueish shadows. Or you could pick a shadow area to be "neutral" and have the warmth of lit areas emphasized by warm tones. Or you could even pick something in between, such as "average" color of the gray card area, to have colors in all the lights slightly off, but overall not emphasized in any general direction.

If you want fully neutral color reproduction, you will have better luck by carrying out your shoot in a studio - where you can control the tonality of all the lights and reflections. On location, you can alleviate the problem of different color temperatures by using your own fill light (with color temperature matched to the main light) to overpower the existing fill light.

Thank you, this answers my question. I suspect that in the forest situation the leafy canopy was reflecting various shades of green, and some objects were in direct sunlight, which made the colors in the picture quite complex. I was looking for a white balance value that would make the leaves, forest ground, rocks and skin tones all look natural, but ended up finding a compromise by manually tweaking it to taste for each picture. A lot of work, but I am satisfied with the results.
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DominikOct 2 '12 at 9:01

I do find that my gray card appears uneven in color even in simpler light situations, though it's usually not as wide a range.
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DominikOct 2 '12 at 9:09

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@Dominki If you ever shoot in a studio or a room with controlled light and plenty of space you'll be amazed at how effortless the white balancing process is and how good the results are. On location there are hundreds of surfaces all bouncing weird coloured light onto your subject at different angles whereas in the studio you just get pure white light.
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Matt GrumOct 2 '12 at 10:55

For white balance uneven light doesn't not matter as white balance seek to adjust compensation used for the RGB (color) buffers. The important thing is to have a color neutral surface, grey card, white paper or anything similar.

The camera then analyze the "white", no matter luminance, to get compensation values for the pictures taken.

Only thing you need to make sure is that when taking WB the current view doesn't over-expose as that leaves everything at max value with nothing to compensate for.

Hi, welcome to the site. The OP is finding that uneven light actually matters, that is by picking different points of his supposedly neutral image of the gray card he should choose wildly different temperatures. This is typical of outdoor situation, as @Imre pointed out in his answer.
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FrancescoOct 2 '12 at 8:43

Then he is doing somehting wrong as WB care not about luminance, only the relation between R, G and B at any luminance. My guess is that he holds the card differently each time, sometime reflects the sky, sometimes indirect, reflected light etc. which would have different "color". Hold the card so it reflect the main light source. That will get you closer to a correct result (or use a Gretach calibration card)..
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Ken FyrstenbergOct 2 '12 at 21:55